Conventionally, a disposable diaper having a front belly part, a back part, and a crotch part has been, for example, known as the worn article described above. In the disposable diaper, the front belly part and the back part are welded to each other by ultrasonic waves through a pair of side seal portions (welded portions).
As a method for forming the side seal portions, there has been employed a method for continuously supplying at least a pair of thermoplastic sheets for constituting the front belly part and the back part to the place between an anvil rotatable about a predetermined shaft and a horn arranged facing the peripheral surface of the anvil (for example, Japanese Unexamined Patent Publication No. H05-15551). The respective thermoplastic sheets are supplied to the place between the anvil and the horn in a state of facing each other.
On the peripheral surface of the anvil, protruding portions are intermittently formed in a circumferential direction. Therefore, the thermoplastic sheets continuously supplied to the place between the anvil and the horn are intermittently welded to each other by ultrasonic waves at a cycle at which the protruding portions come close to the horn. Thus, the mutually-facing thermoplastic sheets are welded to each other to form the welded portions.
According to the producing method described in Japanese Unexamined Patent Publication No. H05-15551, the welded portions are intermittently formed with respect to the continuously-supplied thermoplastic sheets (hereinafter referred to as a continuous supplying method). On the other hand, there has been known a method in which welded portions are continuously formed with respect to intermittently-supplied thermoplastic sheets (hereinafter referred to as an intermittent supplying method).
In order to perform the intermittent supplying method, it is assumed that the thermoplastic sheets are intermittently supplied to the place between an anvil in which the protruding portions are intermittently provided and a horn at a cycle at which the protruding portions come close to the horn in the same way as the continuous supplying method. In this case, however, the following problem arises.
As shown in FIG. 9, a welding force (referred to as a force in some cases) for welding the thermoplastic sheets to each other is generated between the anvil and the horn. The welding force becomes gradually greater and reaches its peak as the protruding portions of the anvil come close to the horn, and becomes smaller as the protruding portions separate from the horn. That is, a time lag is generated until the welding force is increased to a required welding force after the protruding portions of the anvil come close to the horn. Therefore, in the chevron waveform of the welding force, the welding force sufficiently contributes to the welding of the thermoplastic sheets only in a certain period (for example, a period T1 shown in FIG. 9).
Accordingly, in order to obtain sufficient energy (a hatched area in FIG. 9) for the welding, it is required to increase the peak value (power to obtain the value) of the welding force and decrease the supplying rate (the rotating rate of the anvil) of the thermoplastic sheets to increase the period T1.